The primary objective is to relate blood flow and absorption to each other and to contemporary functions in an organ simple enough to permit interpretation, yet perturbed as little as possible from the normal state. A secondary objective is to extend the capability for measurement of organ blood flow under these circumstances, by realizing the potential inherent in the Doppler flowmeter. Observations will be made on the temporarily isolated ventral rumen sac of the conscious cow in order physiologically to differentiate flow in the mucosal and muscular beds, and to study the interrelation of these flows to motility and absorption. Techniques of mucosal flow by clearance methods and organ flow by acoustic methods will be validated. The mechanism by which carbon dioxide enhances urea permeability will be explored.